Re: No power

The old PS does not demonstrate the same effect when initially turned
on, so maybe it is 'dead in a different way'.

That effect, is called "inrush current".

The fact you see that transient, when you flip the switch, means
your primary side capacitor is successfully connected to the line.
You likely have the 300VDC or more, present at the inlet to the
secondary supply circuit. Now the question would be, why isn't the
+5VSB showing up ? Both the +5VSB terminal on the main power cord,
and the PS_ON# signal, should be at 5.0 volts measured value, if
the power supply is just sitting there on the bench.

Here's another bench testing PS data point with a 3rd old PS I found
lying around which I had removed for 'likely faulty' at some time in the
past. I don't remember what was going wrong that caused me to replace
it. We'll call it #3 or 'the really old PS'.

The really old PS also 'bothers' the same circuit when it is switched
on, in fact, it bothers the circuit more than the new broken PS does.

That #3 old PS /does/ run its fan when green/black ps_on#-com is
shorted, which neither of the other PS/s did; and/but it also doesn't
have any voltage at the purple +5vsb pin. I test for that voltage with
the multimeter with one lead in a black com pin and one lead in the
purple +5vsb pin.

I guess I hadn't thrown that suspicious PS away in spite of its being
removed and replaced with a PS which solved whatever the problem was
because I hadn't done any other troubleshooting besides remove and
replace the PS.

I tried the test case here (using an Antec that hasn't blown up yet...)

1) Plug in spare PSU (connecting only its line cord, rest left dangling)
2) Switch ON at the back (switch in "1" position).
3) Check the purple and the green wire, with respect to black.
I got 5.12V on each of those wires, which tells me the
supervisory voltage is present.

In "inrush" dimming of the lights by the Antec, was minimal and
barely noticeable.

The motherboard logic that conditions the front panel momentary
contact POWER switch, cannot work without +5VSB. So it has to be there.
The motherboard logic converts the "pulse" that comes from the
POWER switch, into a steady "level" on PS_ON#. The logic also includes
a four second timer, which is applied when the system has an OS
running, which prevents accidental shutdown by bumping into
the switch. The POWER switch must be held in for four seconds
in that case. And the motherboard logic running from +5VSB,
takes care of that detail. So the POWER switch on the front, doesn't
directly drive PS_ON#. It takes an indirect path, and
the logic in that path needs a power source to work, and
to drive PS_ON#.

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